It's not that he couldn't cast any more powerful magic; he can still cast devastating spells but his borrowed body will suffer the backlash in the real world. Takeru has decided he has no choice but to accept the backlash, no matter how severe it will be this time.Hmmm idk...
This fight has definitely overstayed its welcome, plus it somewhat felt all around the place.
And then Takeru said that his body has reached its limit, and yet he found a way to conjure an even stronger spell? Power of friendship(?) is truly something...
It does it by forcing him to act passively in several previous chapters to force the situation to its current heightened terror level. So, yeah.Thanks for the chapter.
It Beautifully shows THIS is why Takeru needs friends - because no matter how powerful he is, he can't be everywhere at once. He needs partners he can trust to take care of things while he deals with the main threats.